1. Field of the Invention
The present invention relates to an apparatus and a method for optically detecting a plurality of to-be-processed substrates, such as semiconductor wafers or LCD substrates, at one time.
2. Description of the Related Art
A detection apparatus of this type is employed, for example, in a system which is installed in a semiconductor-manufacturing factory so as to clean a large number of semiconductor wafers at one time.
In order to enhance the cleaning efficiency, a semiconductor wafer-cleaning system performs batch cleaning with respect to a number of wafers, e.g., twenty-five wafers. During the cleaning process, the wafers are held by a wafer chuck (i.e., transfer arm) such that they are spaced at regular intervals and their surfaces are kept vertical.
When the wafers are being cleaned, some of them may drop from the wafer chuck. Therefore, the semiconductor wafer-cleaning system incorporates a wafer-detecting apparatus. The wafer-detecting apparatus is generally referred to as a "wafer counter" and examines whether or not any one of the wafers contained in one batch is missing by checking the total number of in that batch. It also examines how the wafers are arranged.
Roughly speaking, a wafer-detecting apparatus adopts one of the following two types of structures:
The structure of the first type employs an optical wafer-detecting means made up of one light-emitting element and one light-receiving element. This optical wafer-detecting means is moved in the direction in which the wafers are arranged, so as to detect the wafers one by one.
The structure of the second type employs a plurality of pairs of light-emitting and light-receiving elements, and the number of the pairs are equal to the number of wafers contained in one batch. The wafer chuck is arranged such that the wafers are partly located in the optical paths between the respective pairs of light-emitting and light-receiving elements. In this state, the wafers are detected.
In the case where the number of wafers of one batch is twenty five, the second structure employs twenty-five pairs of light-emitting and light-receiving elements. The light-emitting and light-receiving elements are arranged alternately and are fixed to a support body by means of mounting plates. Even where two sides of the mounting plates are used, the number of mounting plates required is twenty six, wherein each of the two end plates can be used by its inside, only. Thus, the second structure requires a large number of components. It should also be noted that the mounting plates may be easily shifted from their right positions. If the mounting plates are shifted, the light-emitting and light-receiving surfaces of the paired elements do not become exactly opposite to each other, and the detection sensitivity of one pair becomes different from that of another.
Where twenty-five light-receiving elements are employed in the structure of the second type, twenty-five wafer detection signals will be produced from the output terminals of the light-receiving elements. If these wafer detection signals are output in parallel, twenty-five signal lines and two power supply lines will be required. In other words, either twenty-seven separate lines or a 27-core flat cable will be required. The use of such separate lines or a flat cable inevitably increases the possibility of the occurrence of contact defects or electrical disconnection. In addition, a signal processor is inevitably complicated in structure at the input port.
In order to reduce the number of signal lines required, pairs of light-emitting and light-receiving elements are divided into groups, e.g., five or seven groups, and the light-emitting elements are turned on in units of one group on the time divisional basis. Outputs from the light-receiving elements of each group are transmitted to the signal processor by way of the common terminals of the respective groups. The time-divisional system of this type is advantageous in that it does not require a large number of signal lines.
In the time-divisional system mentioned above, however, it is likely that the light-receiving elements of one group will be undesirably turned on in response to the light from the light-emitting elements of another group, resulting in the generation of error signals. This problem is due to the fact that the light-receiving elements are turned on whenever they receive light whose intensity is greater than the predetermined threshold value.